It is necessary to adjust a rifle scope to a target at a specific distance because a bullet falls as it travels. Thus, a bullet will hit too low on a target when the target is farther away than the distance for which the scope is adjusted, unless the sight is adjusted to compensate for the fall. Most rifle scopes allow the shooter to compensate for the fall of the bullet by adjusting the scope so that the rifle is actually aimed higher when a more distant target is viewed in the cross hairs. The degree to which a rifle is aimed above the horizon is known as elevation, and adjusting the scope to raise the shooting angle is an elevation adjustment. A similar horizontal adjustment allows compensation for a sideways movement of a bullet caused by wind and is called a windage adjustment.
A shooter typically adjusts the elevation of the scope by turning a dial mounted on the scope body. The dial typically is marked with a scale around its diameter, and the body of the scope has an index mark marked on it. The shooter typically adjusts the scope so that a bullet will hit a target at a particular distance, e.g., 100 yards, when the target is in the cross hairs of the reticle. When the shooter wants to shoot at a target that is 200 yards away, he or she rotates the dial a predetermined angular distance so that the cross hairs will be accurately placed for a target at 200 yards. Many scopes use various types of click mechanisms to provide an audible or tactile indication for every increment of rotation. The shooter then does not need to take his or her eye from the target to adjust the elevation. Some scopes also provide a mechanism for adjusting the scale on the dial so that the shooter can align the zero mark of the scale with the index mark on the outer tube when the rifle is sighted in at a reference distance. The shooter can then easily return to the reference sighting or see how much he has adjusted the sight.
Rifle scopes are typically constructed of two tubes: an inner tube and an outer tube. The outer tube is fixed substantially parallel to the rifle barrel. The inner tube is typically fixed at one point to the inside of the outer tube by a ball-type joint, with the free end of the inner tube being movable in both the horizontal and vertical directions. A reticle or an erector lens assembly, or both, can be mounted at the movable end of the inner tube. The movable end can be adjusted up or down to adjust the elevation and horizontally to adjust for windage. Scope designs of this type are described in U.S. Pat. Nos. 3, 297, 389 and 4,408,842 of Gibson.
Many mechanisms have been used or proposed over the years to move the inner tube of rifle scopes to adjust the elevation and adjust for windage. Many such mechanisms use adjusting screws threaded into the outer tube that bear against the inner tube. As the shooter rotates the adjusting screw, it moves further inside the outer tube, pushing the inner tube. A spring located between the inner tube and the outer tube wall opposite the threaded adjusting screws biases the inner tube against the adjusting screws so that when the adjusting screws are rotated so as to move in an outward direction, the inner tube will follow the adjusting screw. The adjusting mechanism may be sealed to the outer tube to maintain within the interior of the outer tube a nitrogen gas charge necessary to prevent fogging and condensation on internal lens surfaces.
In these prior art designs, the rotating adjustment screws bear directly on the inner tube. Any out-of-roundness of the bearing end of the adjustment screw will cause changes to the sighting as the adjustment screw moves. As the windage and elevation adjustments change, the point of contact on the bearing end of the adjustment screw changes. Any deviation of the shape of the outer diameter of the inner tube from that of a perfect cylinder will cause deviations in the adjustment as the adjustment screws push against different points on the inner tube. Moreover, the contact of the adjustment screw end with the inner tube can cause wear on both parts. Wear can make the adjustment less even and can generate small particles that affect the optics and the adjustment mechanism.